As used herein, the following terms have the meanings ascribed. "Potentially superconducting" refers to materials which are known to be superconducting under certain conditions, whether or not those conditions are prevailing at a given time.
The terms "fiber" or "fibers" refer to slender threadlike elongated structures whether in staple or filament form.
The terms "filament" or "filaments" refer to fibers of extreme or indefinite length.
The term "staple" refers to fibers of short, finite length.
The term "spinnable" refers to the property of materials that are capable of large irreversible deformations when subjected to uniaxial stress. Spinnable materials are capable of being formed into fibers.
The term "dry spinning" refers to a process for spinning fibers in which a solution of fiber-forming substance is extruded in a continuous stream to an atmosphere suitable to remove solvent thereby leaving a solid filament.
The terms "superconductor" or "superconductors" refer to materials that will conduct electricity with no loss of energy due to resistance below a certain critical transition temperature (Tc), below a certain critical current density (Jc) and in a magnetic field below a critical strength (Hc). These materials show the Meissner effect (the repulsion of magnetic fields independent of field polarity) below the critical transition temperature (Tc) and critical magnetic field strength (Hc). The critical field strength (Hc) is a function of the temperature. Field strength values are higher at lower temperatures. "High temperature superconductors" are those which have a superconducting transition temperature (Tc) above 77K (boiling point of nitrogen under atmospheric pressure).
The terms "precursor" or "precursors" refer to starting or intermediate materials in the fabrication of the superconducting materials that have not yet been processed into a material capable of superconduction. Metal oxide superconductor precursors are stoichiometric mixtures of nonsuperconducting oxides, nitrates, acetates, carbonates, or other chemical derivatives of potentially superconducting materials that are to be fired or sintered into the superconducting alloy. During firing or sintering, the undesired elements are driven off leaving a superconducting residue.
The term "bicomponent" refers to composite fibers having two distinct non-blended components.
The discovery of high temperature superconductors opened the pathway for several applications (e.g., supermagnets, generators, electrical energy storage). There are, however, many issues that need to be resolved prior to commercial use of the high temperature superconductors. For example, it is believed that, unlike malleable metals, high temperature potentially superconducting materials cannot be processed by first melting and then forming them to the desired shape, for example, wires. One important field of investigation is, therefore, how to produce potentially superconducting wires having sufficient current density and which are sufficiently insensitive to magnetic fields.
Early work along these lines involved mixing a slurry of a superconducting material into a fiber-forming material and wet-spinning the mixture to form fibers. Fibers made by this process generally lack good fiber properties since high loading of superconducting material into the fiber-forming material deteriorates spinning performance. As a result, insufficient superconductor loading resulted in poor superconducting performance, e.g., insufficient consolidation of the superconducting particles. R. B. Cass, "Fabrication of Continuous Ceramic Fiber by the Viscous Suspension Spinning Process", Ceramic Bulletin, Vol. 70, No. 3, 1991 describes the loading of viscose with superconducting material which is then spun.
Oxide ceramic superconducting fibers are described in Japanese Kokai Tokkyo Koho Nos. 01,122,511; 01,122,512; and 01,122,521. The fibers may be made by dispersing the superconductor or its source material in an aqueous solution of a water soluble polymer like polyvinyl alcohol; wet spinning the aqueous solution into a solution which precipitates the polymer; and heating the fibers.
Superconducting fibers based on oxide superconductors and products resulting therefrom may be prepared by extruding the superconductor in a binder. Brazilian Patent Application No. 87 03,412 discloses ceramic oxide powder in a polymer binder. The polymer is removed by heating at 100.degree. C. and the superconductor is sintered.
Japanese Kokai Tokkyo Koho No. 01,176,606 describes a process for making oxide superconducting fiber precursors by dispersing or dissolving an oxide superconductor source material in a solution containing a polymer. The polymer is then spun to form a precursor fiber which is heated. The polymer may be polyvinyl alcohol.
Ceramic superconducting fibers of up to about 200 cm have been spun by drawing nitrate and acetate superconducting precursors in polyacrylic acid gels. Catania, Hovnanian, Cot, "Superconducting YBa.sub.2 Cu.sub.3 O.sub.7-x Fibers From Aqueous Acetate/PAA and Nitrate/PAA Gels", Mat. Res. Bull., Vol. 25, 1990, pp. 1477-1485, describe a lengthwise orientation of the fiber particles. A wet-spinning process is described as preferable. The resulting fibers are described as having poor mechanical properties.
Goto, Sugishita and Kojima, "A New Fabrication Process of Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 Superconducting Filament by Solution Spinning Method Under Ambient Pressure", Physica, C 171, 1990, pp. 441-443 describe the preparation of ceramic superconducting fibers by dry spinning superconducting precursors (yttrium, barium and copper acetates) in a polyvinyl alcohol carrier under one atmosphere oxygen pressure. The resulting fiber was considerably porous.
Goto, "Nonaqueous Suspension Spinning of High-T.sub.c Ba--Y--Cu--O Superconductor", Japanese Journal of Applied Physics, Vol. 27, No. 4, April, 1988, pp. L680-L1682 discusses the nonaqueous suspension spinning of a superconducting ceramic oxide filament by suspending a fine powder of the oxide precursors in a polyvinyl alcohol/dimethyl sulfoxide solution containing a dispersant. The suspension is extruded into a precipitating medium of methyl alcohol and coiled on a winding drum. The wound filament is dried and subjected to heat treatment to generate the superconductor.
European Patent Application Publication No. 0 248 432 discloses a wet-spinning process for making a ceramic green body (which may be a fiber) including contacting a superconductor precursor material slurry with a solidifying liquid. The slurry contains a ceramic powder raw material, a binder and a solvent. The binder may be nitrocellulose or cellulose acetate.
It is known to make ceramic fibers from ceramic precursor sheath/core fibers. U.S. Pat. No. 4,863,799 to Mininni et al. describes a preceramic fiber made by melt, dry or solution spinning a sheath/core fiber in which the preceramic material forms the core. Organosilicon preceramic polymers are spun as a core and certain film forming polymers are used for spinning a sheath layer. Cellulose esters of carboxylic acids, such as cellulose acetate, cellulose propionate, cellulose acetate propionate, and the like, may be used as the sheath.
U.S. Pat. No. 4,559,191 to Arons describes another process for preparing a green ceramic fiber using a sheath/core spinning technique. A green ceramic powder is formed into a dispersion or slurry and placed in the core. Suitable sheath forming polymers include cellulosic esters, among others. When the fiber is wet spun, it is extruded into a coagulation bath. The coagulation bath is any nonsolvent for the sheath forming polymer including water, methanol, propanol, ethylene glycol and the like.
There remains a need for a process to make potentially superconducting filaments having a particle density sufficient to give a usefully high current density and yet still remains manageable, e.g., non-friable.
Superconductor containing fibers have been manufactured using a bicomponent rayon process as described in U.S. patent application Ser. No. 08/198,241 now abandoned and U.S. patent application Ser. No. 08/198,936 now allowed, both assigned to BASF Corporation. One potential disadvantage of using the rayon process to make superconducting fibers is that the superconductor is exposed to several chemicals during this process. For example, the core suspension comes in contact with the viscose in the spin pack.